A method to determine the dissolution and erosion rates of marly gypsum samples from Ripon, UK

Ripon is a town affected by frequent collapse sinkholes that occur due to the rapid dissolution of the underlying gypsum. This gypsum is interbedded and mixed with low solubility but easily water weakened calcareous marl. Construction sites underlain by cavities can be remediated, but if even small flow paths remain, new cavities can appear in close proximity. Dissolution rates previously determined for gypsum have either been on high purity specimens or do not consider the insoluble impurities. It is therefore important to understand the role of interbedded calcareous marls in controlling cavity distribution and growth. A method is proposed to evaluate the effect of marl impurity on gypsum dissolution rates in this area.

For the dissolution test, water is circulated through a hole drilled in a gypsum specimen from Ripon. As the gypsum dissolves, the marl could detach and settle, become suspended or also dissolve. If it remains attached, however, it could impede further gypsum dissolution. Conductivity, total dissolved solids (TDS) and pH of the water are monitored and the test continues until the conductivity has stabilised. This indicates that the water is saturated with gypsum and dissolution has ceased. The water is then evaporated to recover suspended solids, which are put through particle size distribution (PSD) sieves. The post-test specimen mass is added to the recovered solid mass and compared to the pre-test mass.

After testing, mass loss is estimated from both conductivity and TDS curves, and these are compared to measured mass loss. Changes in pH are taken to indicate dissolution of calcareous components in the marl. The conductivity curve is used to find the dissolution rate constant of the specimen, and its cross-section is visually inspected to check the dissolution pattern. The PSD is used to study transport and deposition of insoluble material. Results are combined to assess the influence of marl on gypsum dissolution and sinkhole development, which can be applied both in Ripon and elsewhere.